Relatively thick phosphor storage target having spaced holes for electron passage



April 4, 1967 R. B. McMlLLAN, JR.. ETAL 3,312,850

RELATIVELY THICK PHOSPHOR STORAGE TARGET HAVING vSPACED HOLES FOR ELECTRON PASSAGE Filed Jan. 13, 1964 11 VERT INPUT AMP -42 HORSWEEP GEN. Y v v j RASTER SIG. GEN.

i WRITE +5OQV. WRITE READ READ STORAGE TUBE I I2 J, 45 +50v.

I v F/GZ RICHARD B. McMILLAN RONALD C. ROBINDER INVENTORS BUCKHORN BLORE KLARQUIST 8| SPARKMAN ATTORNEYS 3,312,850 RELATIVELY THICK PHOSPHOR STORAGE TAR- GET HAVING SPACED HOLES FOR ELECTRON PASSAGE Richard B. McMillan, .lTr., Tigard, and Ronald C. Robinder, Beaverton, Oreg., nssignors to Tektronix, Inc., Beaverton, Oreg., a corporation of Oregon Filed Jan. 13, 1964, Ser. No. 337,202 4 Claims. (Cl. 313-68) The subject matter of the present invention relates generally to electron image storage targets and their methods of manufacture, and in particular to direct viewing storage targets in which the storage dielectric includes a layer of phosphor material which may be employed to store a charge image produced thereon and to emit a light image corresponding to such charge image.

Briefly, one embodiment of the storage target of the present invention includes an integral layer of phosphor material supported over a target electrode formed by a light transparent conductive film on the inner surface of the glass face plate of a cathode ray tube. The integral phosphor layer is provided with a plurality of spaced holes extending therethrough from one side of the layer to the other and forming a plurality of separate continuous unobstructed paths through such layer to the conductive film for the secondary electrons emitted from the opposite side of the layer, in order to allow such layer to be of a greater thickness while still enabling bistable storage of the charge image.

The storage target of the present invention is especially useful when employed in the cathode ray tube of a cathode ray oscilloscope to enable such tube to display an electrical signal waveform in a conventional manner or to store such waveform as a bistable charge image for an indefinite controllable time. However, the present storage target can be used in any direct viewing bistable storage tube including those employed in radar and sonar apparatus and in a light image intensifier tube to store the electron image emitted by the photocathode of such intensifier tube onto such storage target.

The storage target of the present invention is an improvement over the phosphor storage target described in copending US. patent application, Ser. No. 180,457, entitled Electron Discharge Display Device which was filed on Mar. 19, 1962, by Robert H. Anderson. The present storage target has all of the advantages over conventional direct viewing storage targets of the mesh or grid control type, which are enumerated in this copending application Therefore these advantages will be mentioned only briefly here and include a more simplified target structure which results from the use of the storage dielectric also as the fluorescent screen of the storage tube by making such dielectric of phosphor material, rather than employing a separate phosphor screen as in the case of conventional direct viewing storage tubes. This simplified structure allows storage targets of larger diameter, improved contrast and greater stable range to be produced in a simpler and less expensive manner.

In addition, by employing an integral phosphor layer having a plurality of spaced openings extending therethrough, the present storage target may be made of a greater thickness so that the brightness of the light image emitted by the phosphor layer is increased accordingly. The present phosphor storage dielectric layer is much thicker since it does not have to be made thin enough to provide a sufiiciently porous structure to enable the secondary electrons to be transmitted through such phosphor layer and collected by the target electrode in the manner of the above-mentioned copending application Ser. No.

180,457, because the spaced openings or holes form a .iU iFCd States Patent Office 3,3l2,850 Patented Apr. 4, 1967 plurality of separate, continuous, unobstructed passageways for accomplishing this without the need of such a porous structure. Since the maximum thickness of the phosphor layer employed as the storage dielectric is no longer critical for bistable storage, the methods of manufacture of the present storage target are considerably simplified in this regard. Thus, fewer defective tar-gets are produced since such targets are not rejected because a thickness of the phosphor layer does not fall within the critical thickness range required by the target disclosed in copending US. patent application Ser. No. 180,457. One of the methods of manufacture of the present storage target is a decalcomania technique similar to that employed in copending US. patent application, Ser. No. 313,091, entitled Method of Manufacture of Phosphor Screen, filed on Oct. 1, 1963, by Herbert J. Mepham, but which also includes the use of two additional organic materials. These two additional materials combine to form a plurality of spaced regions of substantially entirely organic material in the phosphor decal which extend completely through the decal from one side to the other to provide the openings through the phosphor layer left when such organic material is burned out of the decal. Another method of manufacture of the present invention involves a photographic technique similar to that disclosed in copending US. patent application, Ser. No. 299,422, entitled, Storage Target for Cathode Ray Tube and Photographic Method of Manufacture, filed Aug. 1, 1963, by Charles B. Gibson, except that the photosensitive layer of phosphor material employed is exposed in all areas except those corresponding to the openings through the phosphor layer of the storage target. These unexposed regions of the photosensitive layer are removed to provide such openings. This photographic method has the additional advantage that the size and location of the openings may be controlled more readily.

It is therefore one object of the present invention to provide an improved electron image storage target.

Another object of the present invention is to provide a direct viewing storage target in which a layer of phosphor material is employed as the storage dielectric to emit a light image corresponding to the charge image stored thereon over a wide stable range of target voltages.

A further object of the present invention is to provide an improved direct viewing bistable storage target employing an integral phosphor layer of greater thickness as its storage dielectric to increase the brightness of the light image emitted from such layer.

An additional object of the invention is to provide a direct viewing storage target of a simple and inexpensive structure which has a high writing rate and emits a light image having good contrast and resolution.

Still another object of the present invention is to provide a method of manufacturing a direct viewing storage target having an integral layer of phosphor material with a plurality of spaced openings therethrough as its storage dielectric, in a simple and inexpensive manner.

Other objects and advantages of the present invention will be apparent from the following detailed description of certain preferred embodiments thereof and from the attached drawings of which:

FIG. 1 is a diagrammatic view of an electron image storage apparatus including a storage tube having a storage target made in accordance with the present invention, and associated electrical circuitry;

FIG. 2 is a partial horizontal sectional view taken along the line 22 of FIG. 1 showing, on an enlarged scale, a preferred embodiment of the storage target of the present invention; and

FIG. 3 is a view taken along the line 33 of FIG. 2 showing the rear surface of the phosphor layer of the storage target.

A direct viewing bistable storage tube having a storage target .12 made in accordance with the present invention is shown in FIG. 1. This storage tube contains a conventional electron gun including a cathode 14, a control grid 16, and an electrostatic lens structure 18 which may be formed by three focusing and accelerating anodes which operate in a conventional manner to form a narrow beam of the electrons emitted by the cathode. A pair of horizontal deflection plates 20 and a pair of vertical deflection plates 22 are positioned between the electron gun structure and the storage target 12 so that the electron beam emitted by such gun passes between the two plates of each of such pairs of deflection plates before striking the storage target. A separate electron gun may be provided to perform each of the two different functions of producing a charge image on the storage target by means of a writing beam of high velocity electrons, and for producing an electrical readout signal corresponding to such charge image by scanning the target with a reading beam of lower velocity electrons. However, it is also possible to accomplish these two functions by means of a single electron gun. This is done in FIG. 1 by changing the position of each of three ganged switches 24, 26 and 28 connected, respectively, to the control grid 16, the horizontal deflection plates 20 and the vertical deflection plates 22 between WRITE and READ positions, in a manner hereafter described.

One or more flood guns 30 may be mounted within the envelope of the storage tube 10 to bombard the surface of the storage target substantially uniformly with low velocity flood electrons in order to maintain or hold the charge image produced on such storage target by the writing beam by means of secondary electron emission. This causes bistable storage of such image for an indefinite controllable time.

As shown in FIGS. 2 and 3, the storage target 12 includes a target electrode 32 in the form of a thin light transparent coating of conducting material, such as tin oxide, on the inner surface of a flat, glass face plate 33 at one end of the tube envelope. The storage dielectric of the target is an integral layer 34 of phosphor material supported over the target electrode 32 so that such phosphor layer performs the dual functions of storing a charge image produced thereon and of emitting a light image corresponding to such charge image. This light image is transmitted through the target electrode 32. and the face plate 33 for direct viewing. The phosphor layer 34 is provided with a plurality of spaced openings 36 which extend completely through the layer from one side to the other to provide a plurality of separate continuous unobstructed paths through such layer to the target electrode 34 for secondary electrons emitted from the opposite side of the layer. These openings enable the thickness of the phosphor layer to be increased beyond that previously thought to be the maximum thickness for integral phosphor layers employed as storage dielectrics for bistable storage targets because the secondary electrons are transmitted through such openings, not through the pores between the phosphor particles, to the target electrode 32. Thus the phosphor layer 34 is of too great a thickness to provide a sufficiently porous structure to enable secondary electrons to be transmitted through the pores between the phosphor particles to the target electrode 32. As a result of the greater thickness of hte phosphor layer 34, the light image emitted from such layer is of a greater brightness. Since the maximum thickness of the phosphor material is no longer critical, it is only limited by the amount of phosphor material which will adhere to the face plate.

The target electrode 32 is connected to a DC. target voltage produced across fixed load resistor 37 by a DC.

4 current flowing through a variable resistor 38 from a source of positive D.C. supply voltage of about +500 volts, as shown in FIG. 1. The setting of variable resistor 38 determines the amount of current flowing to ground through the fixed resistor 37 and controls the voltage drop across such fixed resistor. When the target voltage is approximately +200 volts with respect to the grounded flood gun cathode it is within the stable range of target voltages over which the phosphor layer 34 stores a bistable charge image for an indefinitely controllable time. Then the storage tube 1% can be operated in a storage mode by moving the switches 24, 26 and 28 to the WRITE position. This connects the horizontal deflection plates to a horizontal sweep generator 40 through switch 26 and connects the vertical deflection plates 22 to a vertical amplifier 42 through switch 28. Thus, when an input signal is applied to the input terminal 44 of the vertical amplifier it is transmitted to the vertical deflection plates. The input signal may also be employed to trigger the horizontal sweep generator so that the writing electron beam emitted by cathode 14 is deflected horizontally as well as vertically to produce a charge image of the input signal waveform on the phosphor layer 34 of the storage target.

The writing beam contains high velocity electrons due to the fact that cathode 14 ,is connected to a high negative DC. voltage source of about 3,000 volts, and therefore produces a charge image on the phosphor layer 34 by secondary electron emission which is more positive than target areas not struck by such beam. When the potential of the written charge image is above the first cross over voltage of the secondary emission characteristic curve of the phosphor layer, the low velocity flood electrons emitted by the grounded cathode of flood guns 30 drive the potential of the charge image to a high voltage stable state near the potential of the target electrode. At the same time these flood electrons drive the potential of the unwritten background areas of the phosphor layer initially charged below the first cross over voltage, to a low voltage stable state near the voltage of the flood gun cathodes. One or more wall band electrodes 45 are coated on the inner surface of the funnel portion of the storage tube envelope to focus the flood electrons uniformly onto the phosphor layer 34 of the storage target. This wall band electrode may be connected to a source of positive D.C. supply voltage of about +50 volts when the flood gun cathode is grounded. This conventional bistable storage operation has been described in greater detail in copending US. patent application, Ser. No. 180,457, referred to above, and for this reason will not be discussed further here.

In order to erase the charge image stored on the phosphor layer 34 of the storage target, the voltage applied to the target electrode 32 is increased above the fade positive voltage of the phosphor merely by varying the value of resistor 38 or by applying a positive voltage pulse to such target electrode. The target voltage is held above this fade positive voltage for a sutficient time to cause the potential on the rear surface of the phosphor layer to become uniformly equal to a potential near the voltage on the target electrode. Then the target voltage is decreased below the retention threshold voltage, also referred to as the first cross over voltage, for the hosphor layer below which the storage target cannot store a bistable charge image. The target voltage is held below the retention threshold voltage until the flood electrons cause a potential on the rear surface of the phosphor layer to be driven down to a uniform potential near the voltage on the flood gun cathode. Then the voltage applied to the target electrode 32 is gradually incresed above the retention threshold voltage to a target volt-age within the stable range without causing the potential of the rear surface of the phosphor layer to follow such target voltage by capacitive coupling action. The erasure of the charge image is now complete and the target is ready to receive another charge image.

The phosphor layer 34 of the storage target 12 may be made of P-l type phosphor material which is zinc orthosilicate activated by manganese. However, other phosphors will also store charge images such as P-3 type phosphor (zinc beryillium orthosilicat-e) and P-5 type phosphor (calcium tungstate). In addition, high resistivity phosphors, such as zinc sulfide activited by manganese, have been employed successfully. In order to increase the writing rate of the phosphor layer .34, it may also be desirable to add a good secondary emissive material, such as magnesium oxide, to the phosphor material. Thus, such phosphor layer may contain a small amount of secondary emissive material which may be between and 25 in the case of a layer of magnesium oxide and P-1 type phosphor. In addition to increasing the writing rate, it also appears that the presence of the magnesium oxide increases the usable lifetime of the storage target. Of course too great a percentage of purely secondary emissive material will decrease the brightness of the light image. emitted by the phosphor layer appreciably because phosphor particles are replaced by non-luminescent secondary emissive material.

The performance of the present storage target is effected by the size of the openings 36 through the phosphor layer, as well as by the number of such openings per unit area. This is apparently due to the fact that the secondary electrons emitted from the phosphor layer due to the bombardment of such layer by the writing beam and by the flood electrons, are transmitted through such openings and collected by the target electrode 32. Thus, the size of the openings must not be too small or the number of openings per unit area too few to prevent eflicient collection of the secondary electrons by means of the target electrode. However, such openings cannot be too large or the number of openings per unit area too great because this would decrease the brightness of the light image emitted by the phosphor layer and would also cause loss of image resolution. It has been found that for a phosphor layer containing substantially 100% P-l type phosphor and having a thickness between .0015 and .003 inch, the openings 36 should have a diameter between .0006 and .002 inch and should cover a surface area of between 10% and 25% of the phosphor layer in order to phor layer is, the more phosphor material in such layer so that more magnesium oxide may be employed and still maintain the brightness level the same as before.

While scan conversion or electrical readout is not necessary when employing a direct viewing storage tube, it is sometimes convenient to produce an electrical readout signal corresponding to the charge image stored on the target. This may be accomplished with the tube of FIG. 1 simply by rotating the movable contacts of switches 24, 26 and 2-8 to the READ position in order to connect the control grid 16 to a more negative DC. voltage of about -3,050 volts when the cathode 14 is connected to a negative DC. voltage of 3,000 volts. This reduces the current density of the reading beam of electrons emitted by such cathode and transmitted through the lens structure 18 to the storage target 12 in order to prevent such reading beam from producing a stored image on the target during electrical readout. The same result may also be accomplished by reducing the negative potential applied to the cathode during readout, while maintaining the current density the same, in order to decrease the velocity of the electrons in the reading beam. In addition, the horizontal deflection plates 20 and the vertical deflection plates 22 are both connected by switches 26 v and 28, respectively, to a raster signal generator 46.

The raster signal generator 46 applies conventional saw tooth sweep'signals of different frequencies to the vertical and the horizontal deflection plates in order to scan the reading beam over the storage target in a conventional television raster pattern. As a result, an electrical readout signal is produced upon the target electrode 32 of the storage target 12. This electrical readout signal is transmitted through a coupling capacitor 48, a low impedance preamplifier 50 and a high gain voltage amplifier 52 connected in that order, to the Z-axis input of a remotely positioned television monitor tube 54 or other suitable recording device. The horizontal and vertical deflection plates of the monitor tube 54 are also connected to the raster signal generator 46 so that the raster signals applied thereto are similar to those applied to the horizontal and vertical deflection plates of the storage tube. Since the electrical readout signal is applied to the cathode or control grid, either of which may function as the Z-axis input of the TV monitor tube 54, the electron beam within such tube is intensity modulated in accordance with the electrical readout signal to form a reproduction of the stored signal waveform as a television picture on the fluorescent screen of such monitor tube.

As indicated above, the target electrode 32 may be provided as a thin, light transparent conductive coating on the inner surface of the flat rectangular glass face plate 33 of the tube envelope with a lead portion 56 of this target electrode extending through a seal between such face plate and a funnel portion 58 of the tube envelope. The funnel portion may be made of ceramic material and sealed to the face plate by means of a suitable glass frit 60 in the region surrounding the phosphor layer 34. In this manner the target voltage produced across resistor 37 may be applied to the target electrode '32 by connecting lead portion 56 to such resistor on the exterior of such envelope. Also, an internal graticule scale 62 in the form of a plurality of notches or deposited lines of glass frit or other suitable light reflecting material may be provided on the inner surface of the face plate 33 beneath the target electrode 32 in order to measure the characteristics of the signal waveform images stored on the storage target. This internal graticu'le scale may be edge lighted by projecting light through the surrounding outer edge of the face plate 33 to illuminate such scale.

There are several methods of making the storage target structure of FIGS. 2 and 3. One of these methods involves a decalcomania technique which includes the step of first preparing a liquid mixture of phosphor material, such as P-l type phosphor, an organic binder, such as methyl methacrylate or the thermoplastic resin sold by Du Pont de Nemours and Co. under the name ELVAX 250, and a solvent, such as toluene, benzene or tetrahydrofuran for such binder in a similar manner to that described in copending US. patent application, Ser. No. 313,091, referred to above. To this mixture is also added a pair of organic materials having two hydroxyl groups and a lower evaporation rate than the solvent, including a first organic material such as triethanolamine, glycerin, propylene glycol, or ethylene glycol, and a second organic material which combines with the first organic material to separate such two organic materials from the remainder of the mixture into substantially pure regions of organic material containing no phosphor. This second organic material may be a particulate material, such as sugar or starch, or it may be a non-particulate material such as glycerin, ethylene glycol, erythritol, arabitol, or mannitol. Sucrose sugar and triethanolamine have been used together as the two organic materials with good success, as well as glycerin and triethanolamine.

In one specific embodiment of the decal technique, 50 grams of ELYAX 250 resin binder, which is a copolymer of polyethylene and polyvinyl acetate, were mixed with 1,000 milliliters of toluene to provide a carrier solution, Then 200 milliliters of carrier solution is 7 mixed with 4 milliliters of butyl alcohol, 8 drops of triethanolamine, 84 grams of Pl type phosphor material and 12 grams of sucrose sugar. It should be noted that the butyl alcohol functions as a wetting agent for the casting plate on which the decal is formed. The resulting slurry is then stirred for several hours to throughly mix the materials so that the phosphor and sugar are uniformly distributed throughout the slurry. In addition, 10.6 grams of magnesium oxide may be added to the slurry if it is desired to provide a phosphor layer 34 which is approximately magnesium oxide. It should be noted that medium sized phosphor particles and smaller provide a more uniform thickness phosphor layer and they may be obtained by water settling the larger phosphor particles out of the phosphor material before adding it to the slurry.

The slurry is then spread in a thin layer having a thickness in the range of about .0015 to .006 inch on a smooth flat casting plate of glass by means of a doctors knife. The spacing of this knife from the surface of the casting plate is approximately twice the desired thickness of the liquid layer to be applied to such plate. Immediately after filling the reservoir of the doctors knife, such knife is drawn over the casting plate at a substantially constant speed of approximately two feet per second. The liquid layer is then allowed to dry for about 30 minutes to evaporate the toluene solvent and form a solid film of resin binder, phosphor, magnesium oxide, sugar and triethanolamine. Next, a decal is cut in the solid film on the casting plate. The shape of the decal is determined by the configuration of the face plate 33 of the cathode ray tube envelope. Next, the decal is removed from the casting plate by soaking it in water to loosen such decal and contacting it with a paper towel or a piece of fine filter paper which has been soaked with water. The paper is laid in contact with the decal for about one minute and then removed from the casting plate to strip the decal off the casting plate due to its adherence to the paper.

The decal is then applied to the face plate 33 over the tin oxide coating of the target electrode 32 which has previously been applied in a conventional manner on one side of such face plate. When the organic binder used to make the decal is ELVAX 250, the decal is pressure sensitive so that it will stick to the face plate quite easily and is held there temporarily by such adhesive. However, when methyl methacrylate is employed ethyl alcohol and butyl alcohol may be applied to the decal to soften the resin in order to insure that the decal adheres to the face plate with no air bubbles beneath such decal. Next, the decal is dried to remove any water on such decal. It should be noted that at this time the decal contains the resin binder through which the phosphor material and magnesium oxide are uniformly disbursed along with some spaced regions of triethanolamine and sugar which have combined together and separated from the remainder of the liquid layer applied to the casting plate before such layer is dried into a solid film. Each of these spaced regions of triethanolamine and sugar form spaced portions of substantially entirely organic material which extend entirely through the decal from one .side to the other. However, much of the sugar and triethanolamine is washed out of the decal when it is immersed in water during the removal step, because these materials are soluble in water.

The final step in the decal method involves heating the decal supported on the face plate in an oven to a temperature of about 500 degrees Centigrade by slowly increasing the temperature in the oven from room temperature to 500 degrees Centigrade in about two hours, maintaining it at 500 degrees centigrade for about 30 minutes, and then slowly decreasing the temperature of the oven back to room temperature at slightly slower rate than it was raised. This heating step is carried out in an oxygen containing atmosphere in order to completely decompose organic material of the resin binder, the triethanolamine and the sugar by oxidation without leaving any appreciable ash or other residue. This causes a layer 34 consisting substantially entirely of phosphor particles and magnesium oxide to remain adhered to the surface of the face plate in the form of the porous structure shown in FIGS. 2 and 3. The pores or openings 36 extending through this phosphor layer are located in place of the regions of combined triethanolamine and sugar which extended completely through the decal so that they left such openings after such regions were oxidized.

Another method of making the storage target 12 of the present invention involves the use of a photographic technique to produce the openings 36 through the phosphor layer 34. This method includes preparing a liquid slurry of phosphor particles and photosensitive material which may be polyvinyl alcohol and an activator of ammonium dichromate. The photosensitive solution may contain the following proportions: grams of polyvinyl alcohol, 1000 milliliters of water, 1 milliliter of isoproponyl and 20 grams of ammonium dichromate. This photosensitive solution is then mixed with the phosphor material in a proportion of 10 grams of phosphor for 100 milliliters of photosensitive solution. The resulting phosphor slurry is then applied in a thin layer to the inner surface of the face plate over the transparent conductive film of the target electrode 32. The liquid layer of phosphor and photosensitive material is then exposed to a light image in the form of a plurality of spaced dot shadows corresponding to the openings 36 in the phosphor layer 34. This may be accomplished by positioning a photographic film negative of the dots adjacent the front surface of the face plate and transmitting light through the film negative around the opaque dots to project such dots onto the photosensitive layer. This causes the exposed portions of the photosensitive layer to harden to a depth determined by the exposure time. However, the unexposed portions of the photosensitive layer remain in a liquid state, corresponding to the dots on the film negative.

Next, the surface of the coated face plate is washed with water to remove the unexposed dot portions of the photosensitive layer without removing the exposed regions of such layer. This provides a solid layer of hardened exposed photosensitive material and phosphor material having a plurality of openings 36 therethrough. Of course, the liquid layer of photosensitive material and phosphor may be allowed to dry to a solid film before exposure to prevent any possible movement of the layer during exposure. However, then it takes longer to remove the unexposed dot portions.

If magnesium oxide is employed as a secondary emissive in the phosphor layer, it may be desirable to use a different photosensitive material because the dichromate activator of the polyvinyl alcohol attacks the magnesium oxide to some extent, but this is not absolutely necessary. Also, this may be avoided by applying the magnesium oxide in separate solution over the surface of the photosensitive layer after it has been exposed to the light image, to allow the magnesium oxide solution to soak into the exposed and unexposed portions of the photosensitive layer. Then it is only necessary to wash out the unexposed polyvinyl alcohol in order to remove the magnesium oxide contained therein and leave a layer of hardened photosensitive material containing phosphor papticles and magnesium oxide with a plurality of spaced holes therethrough. Finally, the coated face plate is heated in an oven to a temperature of about 400 degrees Centigrade for about 30 minutes in an oxygen atmosphere to remove the organic photosensitive material from the phosphor layer by oxidation without leaving any appreciable ash.

An alternative photographic method may be employed in which the phosphor material is not provided in the photosensitive solution, but instead is water settled onto the conductive coating of the face plate through a mask in the form of a plurality of spaced dots provided by th photosensitive layer. In this case the photosensitive layer is first exposed to a positive light image of the dots so that unexposed areas surrounding the dot areas are removed to form the mask. After the phosphor is applied, the mask dots are removed by the subsequent heating step to leave a plurality of spaced openings through the phosphor layer.

It should be noted that the film negative is spaced from the photosensitive layer by the thickness of the face plate when such negative is positioned in contact with the outer surface of the face plate. Thus, the light image of the dots on such negative may be projected in such a manner that the unexposed regions on the photosensitive layer corresponding to such dots are distorted due to parallax. This may be prevented to some extent by employing a colluminated light source. lowever, it may also be desirable to provide a plurality of spaced light opaque dots (not shown) of aluminum or silver beneath the photosensitive layer. These dots then function as a light opaque mask in place of the film negative for exposing the light sensitive layer in a similar manner. The masking dots would then become a permanent part of the storage target and also serve the additional purpose of increasing the contrast of the light image emitted from the phosphor layer 34 by covering the holes 36 in such phosphor layer when the storage target is employed to store a charge image thereon. This photographic technique has the additional advantage that the size of the holes 36 as well as the spacing of such holes may be easily controlled merely by changing the pattern of the film negative or by applying the silver masking dots as described above.

It will be obvious to those having ordinary skill in the art that various changes may be made in the details of the above described preferred embodiments of the present invention without departing from the spirit of the invention. Therefore, the scope of the invention should only be determined by the following claims.

We claim:

1. A direct viewing electron image storage apparatus, comprising:

a light transparent support plate of electrical insulative material;

a light transparent electrical conductive film coated on one side of said support plate;

a secondary emissive storage dielectric layer of phosphor material supported on said support plate over said conductive film, said layer being formed as an integral layer of phosphor particles with a plurality of spaced holes extending completely through said layer from one side to another to provide a plurality of separate continuous unobstructed paths for secondary electrons to be transmitted from the bombarded side of said layer to said conductive film on the opposite side of said layer;

said phosphor layer being of too great a thickness to provide a sufliciently porous structure to enable said secondary electrons to be transmitted through the pores between the phosphor particles to said conductive film;

writing means for bombarding the phosphor layer with high velocity Writing electrons to form a charge image on said layer; and

holding means for bombarding said phosphor layer with low velocity holding electrons to cause secondary electrons to be emitted from said phosphor layer and bistable storage of said charge image.

2. A direct viewing bistable electron image storage tube, comprising:

an envelope including a light transparent face plate of electrical insulative material;

a light transparent electrical conductive film coated on one side of said face plate;

a secondary emissive storage dielectric layer of phosphor material supported on said support plate over said conductive film, said layer being formed as an integral layer of phosphor particles with a plurality of spaced holes extending completely through said layer from one side to another to provide a plurality of separate continuous unobstructed paths for secondary electrons to be transmitted from the bombarded side of said layer to said conductive film on the opposite side of said layer;

said phosphor layer being of too great a thickness to provide a sufiiciently porous structure to enable said secondary electrons to be transmitted through the pores between the phosphor particles to said conductive film;

Writing means for bombarding the phosphor layer with high velocity writing electrons to form a charge image on said layer;

holding means for bombarding said phosphor layer substantially uniformly with low velocity flood electrons to cause secondary electrons to be emitted from said layer and store said charge image as a bistable charge image for an indefinite controllable time; and

readout means for scanning said phosphor layer with an electron beam to produce an electrical readout signal on said conductive film corresponding to the charge image stored on said phosphor layer.

3. A bistable storage apparatus, comprising:

a support member having an electrically conductive surface; 7

storage dielectric means including an integral layer of phosphor material supported on said conductive surface of said support member, said phosphor layer having a plurality of spaced holes extending completely through said layer to provide a plurality of separate continuous unobstructed passageways for secondary electrons to be transmitted from the bombarded side of said layer and collected by said conductive surface on the opposite side of said layer;

said phosphor layer being of too great a thickness to provide a sufiiciently porous structure to enable said secondary electrons to be transmitted through the pores between the phosphor particles to said conduc tive surface;

writing means for forming a charge image on said phosphor layer; and

holding means for bombarding said phosphor layer with low velocity electrons to cause secondary electrons to be emitted from said phosphor layer and to enable bistable storage of said charge image.

4, A storage apparatus in accordance with claim 3 in which the total cross sectional area of said holes is between about 10 and 25 percent of the area of said phosphor layer.

References Cited by the Examiner UNITED STATES PATENTS 2,777,087 1/1957 Fromm 3l5-l2 X 2,839,679 6/1958 Harris 315-l2 X 2,843,799 7/1958 Hook et al. 3l5-12 JAMES W. LAWRENCE, Primary Examiner. R. SEGAL, Assistant Examiner. 

3. A BISTABLE STORAGE APPARATUS, COMPRISING: A SUPPORT MEMBER HAVING AN ELECTRICALLY CONDUCTIVE SURFACE; STORAGE DIELECTRIC MEANS INCLUDING AN INTEGRAL LAYER OF PHOSPHOR MATERIAL SUPPORTED ON SAID CONDUCTIVE SURFACE OF SAID SUPPORT MEMBER, SAID PHOSPHOR LAYER HAVING A PLURALITY OF SPACED HOLES EXTENDING COMPLETELY THROUGH SAID LAYER TO PROVIDE A PLURALITY OF SEPARATE CONTINUOUS UNOBSTRUCTED PASSAGEWAYS FOR SECONDARY ELECTRONS TO BE TRANSMITTED FROM THE BOMBARDED SIDE OF SAID LAYER AND COLLECTED BY SAID CONDUCTIVE SURFACE ON THE OPPOSITE SIDE OF SAID LAYER; SAID PHOSPHOR LAYER BEING OF TOO GREAT A THICKNESS TO PROVIDE A SUFFICIENTLY POROUS STRUCTURE TO ENABLE SAID SECONDARY ELECTRONS TO BE TRANSMITTED THROUGH THE PORES BETWEEN THE PHOSPHOR PARTICLES TO SAID CONDUCTIVE SURFACE; WRITING MEANS FOR FORMING A CHARGE IMAGE ON SAID PHOSPHOR LAYER; AND HOLDING MEANS FOR BOMBARDING SAID PHOSPHOR LAYER WITH LOW VELOCITY ELECTRONS TO CAUSE SECONDARY ELECTRONS TO BE EMITTED FROM SAID PHOSPHOR LAYER AND TO ENABLE BISTABLE STORAGE OF SAID CHARGE IMAGE. 